Group III-N compounds, such as gallium nitride (GaN) and its related alloys have been under intense research in recent years due to their promising applications in electronic and optoelectronic devices. Particular examples of potential optoelectronic devices include blue light emitting and laser diodes, and UV photodetectors. Their large bandgap and high electron saturation velocity also make them excellent candidates for applications in high temperature and high-speed power electronics.
Due to the high equilibrium pressure of nitrogen at typical growth temperatures, it is extremely difficult to obtain GaN bulk crystals. Owing to the lack of feasible bulk growth methods, GaN is commonly deposited epitaxially on substrates such as SiC and sapphire (Al2O3). However, a current problem with the manufacture of GaN thin films is that there is no readily available suitable substrate material which exhibits close lattice matching and close matching of thermal expansion coefficients.
SiC is a semiconducting material which provides excellent thermal conductivity, but is very expensive and available only in small wafer sizes. Direct growth of GaN on SiC is generally difficult due to poor wetting between these materials. Although buffer layers, such as AlN or AlGaN, can be used to address this wetting problem, such layers increase the resistance between the device and the substrate. In addition, it is very difficult to prepare a SiC layer having a smooth surface. A rough interface with GaN can cause an increase in defect density of the GaN layer.
Presently, (0001) oriented sapphire is the most frequently used substrate for GaN epitaxial growth due to its low price, availability of large-area wafers with good crystallinity and stability at high temperatures. However, the lattice mismatch between GaN and sapphire is over 13%. Such a huge mismatch in the lattice constants causes poor crystal quality if GaN films were to be grown directly on the sapphire, due to stress formation and a high density of defects, including such defects as microtwins, stacking faults and deep-levels. Sapphire is also an electrical insulator. Use of electrically insulating substrates can complicate processing by requiring additional processing steps, as compared to a conducting or semiconducting substrate, due to the inability to make an electrical contact through the substrate.
The most highly refined semiconductor substrate in the world are silicon wafers. Silicon is increasingly being used as a substrate for GaN materials. Silicon substrates have been considered for use as substrates for growth of GaN films. Silicon substrates for GaN growth is attractive given its low cost, large diameter, high crystal and surface quality, controllable electrical conductivity, and high thermal conductivity. The use of Si wafers promises easy integration of GaN based optoelectronic devices with Si based electronic devices.
The disadvantages of Si as a substrate for GaN heteroepitaxy include a +20.5% a-plane misfit which initially led to the conclusion that growth of GaN directly on silicon was not likely to work well. In addition, the thermal expansion misfit between GaN (5.6×10−6 K−1) and Si (6.2×10−6 K−1) of 9.6% can lead to cracking upon cooling in films grown at high temperature. Thus, direct growth of GaN on substrates including Si has been found to result in either polycrystalline growth, substantial diffusion of Si into the GaN film and/or a relatively high dislocation density (e.g. 1010 cm−2). Moreover, GaN is also known to poorly nucleate on Si substrate, leading to an island-like GaN structure and poor surface morphology. Thus, the quality of GaN films grown on silicon has been far inferior to that of films grown on other commonly used substrates such as sapphire or silicon carbide. Moreover, the growth conditions that have been used for GaN on Si are generally not compatible with standard silicon processes.
Numerous different buffer layers have been disclosed for insertion between the Si substrate and the GaN layer to relieve lattice strain and thus improve GaN crystal quality. However, even when buffer layers are used, typically the effect of the thermal expansion coefficient mismatch is too large to suppress the formation of cracks in the GaN and related other Group III-N films grown. Thin AlN, GaAs, AlAs, SiC, SiO2, Si3N4 and ZnO, boron monophosphide (BP) or low-temperature GaN layers are exemplary buffer layers which have for GaN growth on Si.
BP has a zinc blende crystal structure with a lattice constant of 4.5383 Å at room temperature. The lattice mismatch between GaN and BP is less than 0.6%. BP has 16% lattice mismatch to Si, but boron and phosphorous have a high affinity to Si as evidenced by their use as common dopants in bulk Si. BP is a stable material. The sublimation temperature of BP is greater than 1130° C. Physical properties of GaN, BP and Si are given in Table 1.
TABLE 1Physical properties of GaN, BP and SiPropertyGaNBPSiCrystal StructureWurtziteZincblendeDiamondLattice Constant (Å)a = 3.2104.5375.431c = 5.237Band Gap (eV)3.42.31.1Band TransitionDirectIndirectIndirectConductivity Typesp, np, np, nElectrical Resistivity10−3 − 10−21012105(Ω − cm)Density (g/cm3)6.102.402.33Hardness (GPa)N/A30–3511Melting Temperatures (° C.)78211301410(sublimates at 1 atm)(decomp. to B13P2)Thermal Expansion5.64.02.9Coefficient (K−1 × 10−6)Thermal Conductivity1.33.51.5(K−1)
BP is known as a buffer material for Si(100) substrates. However, GaN grown on BP on single crystal Si (100) has been mainly cubic, with significant evidence of mixed phases. For example, Nishimura, et al., Growth of c-GaN on Si (100) Materials Science and Engineering B82 (2001) 25–26 reports mainly cubic mixed phase GaN on BP/Si (100).
BP has not generally been reported to be a buffer material for III-nitride semiconductors, such as GaN, on Si(111). However, U.S. Pat. No. 6,069,021 to Terashima et al. discloses formation of a boron phosphide-base semiconductor layer using a silicon single crystal having a surface of either a {100} or {111} crystal plane as the substrate. This reference teaches that BP directly grown on a Si single crystal substrate at a temperature exceeding 700° C. becomes a discontinuous layer of pyramid-like BP crystal islands scattered over the silicon surface. It is concluded therein that a single BP layer formed at a temperature higher than 700 C is therefore not effective as a buffer layer for GaN growth on Si.
To overcome the lattice mismatch between {111} crystal planes of the silicon single crystal and that of BP, a two buffer layer stack is disclosed in U.S. Pat. No. 6,069,021. The first buffer layer deposited on the Si substrate is disclosed as being polycrystalline or amorphous and is said to provide strain relief. A second BP buffer layer is referred to as being “single crystal” and is deposited on the first buffer layer.
In another approach, U.S. Pat. App. No. 20030047795 discloses a semiconductor device having a silicon single crystal substrate and a {110} boron phosphide semiconductor layer containing boron and phosphorus as constituent elements on a surface of the silicon single crystal substrate. The surface of the silicon single crystal substrate is a {111} crystal plane inclined at an angle of 5.0 to 9.0 degrees toward a <110> crystal azimuth. However, as with U.S. Pat. No. 6,069,021, the BP buffer layer includes an amorphous portion, such as a major part being amorphous, such as disclosed in Example 1.